3D-printed integrative probeheads for magnetic resonance

The study addresses the challenge of fabricating MR radio frequency probeheads that are integrated, customizable, and miniaturized. Conventional coils made by manual winding or PCB lithography are labor-intensive, largely 2D, and imprecise for complex 3D geometries, limiting performance and fill factor, especially for microliter-scale samples and microfluidic in situ experiments. The authors propose an integrative fabrication approach combining high-precision 3D printing with liquid metal (LM) infusion to create monolithic probeheads that embed RF coils, customized sample chambers, and RF interfaces with micrometer-scale precision. This aims to improve SNR via better filling factor and enable unconventional MR experiments such as in situ electrochemistry, continuous-flow reaction monitoring with paramagnetic particle/ion separation, and small-sample MRI. The work also tackles material constraints (dielectric, magnetic susceptibility, background signals) and non-capacitive coil design for high-frequency operation.

3D printing (additive manufacturing) offers rapid prototyping and complex 3D microstructures used across biomaterials and microelectronics, but faces challenges for MR probeheads due to specific electromagnetic and magnetic susceptibility requirements. Prior related work includes 3D-printed phantoms/supports and printed electronics, and isolated examples of monolithically integrated coils or LM-filled RF structures; however, comprehensive integrative MR probeheads combining 3D-printed structures with LM coils have not been explored. Material properties such as low dielectric constant and loss tangent, diamagnetism, and minimal background signals are critical for MR performance. Existing custom coil technologies include copper coils (high Q), inkjet-printed silver paste coils, microcoils, and flexible printed coils, each with trade-offs in Q factor, fabrication complexity, and adaptability.

Design and simulation: Multiple coil and sample chamber geometries (solenoid, saddle, modified Alderman-Grant) were designed in CAD (SolidWorks) and simulated in CST Microwave Studio to optimize RF B1 field strength and homogeneity for a Varian 11.7 T system at 500 MHz. Due to integrative constraints, non-capacitive volumetric coil types were considered. Coil channels had 3 mm inner diameter and 400 µm thickness to ease cleaning and injection. Simulations (PEC conductors) indicated saddle coils outperform modified AG coils in Q, homogeneity, and normalized SNR, leading to saddle coil selection for NMR applications. 3D printing and fabrication: Probeheads were printed as monolithic transparent polymer blocks embedding coil channels, sample chambers/microfluidics, and RF interfaces. Two printing technologies were used: FDM (ProJet 3510SD; 30 µm resolution) and SLA (Cyclone W-1 and SLA300 DLC; 25 µm resolution). Dual-nozzle printing formed structural bodies and sacrificial materials to create hollow channels. Post-printing, LM pastes were infused through injection ports to form conductive coils; copper strips served as feedlines to matching circuits. Injection ports and interfaces were sealed with silver paste and epoxy. Ultrasonic vibration removed trapped air bubbles. LM pastes were solidified to stabilize conductors. Matching networks were attached to complete probes. Building material selection: Dielectric properties (εr, tanδ) of candidate printing materials were measured in MR frequency ranges using resonator-based techniques (five samples per material to average errors). Materials tested included VisiJet M3 Crystal (FDM), Formula L1, Clara A (SLA300), and PT-series resins (Cyclone W-1). Unloaded Q factors of LM-filled saddle probeheads made from each material were measured with an Agilent E5071C. Considering dielectric properties, Q, diamagnetism, hydrogen content, and background spectral baseline, PLA-equivalent VisiJet M3 Crystal was chosen. Table 1 reports εr/tanδ and corresponding unloaded Q: VisiJet M3 Crystal εr=2.68, tanδ=0.012, Q=45 (best among tested). PTFE was a reference, not printable. LM paste formulation and characterization: Low-toxicity Ga-based LMs (Ga, EGaIn) were selected. To increase conductivity, uncoated metal microparticles were dispersed in Ga via high-energy sonication after acid treatment to remove oxide skin; dispersed oxides help suspend particles. Au microparticles (10 µm) were mixed at 0–5 wt% in 2 g batches. Conductivity was measured using I-shaped molds across replicates. Conductivity increased up to 3 wt% Au, reaching 3.82×10^6 S m^-1 (~10% above pure Ga), then decreased at higher loadings, likely due to increased contact resistance and non-uniform dispersion under molding pressure. Temperature dependence (293–308 K) showed <5% conductivity variation. Solid-state LM operation was preferred; mixed particles acted as nucleation sites, improving solidification and thermal stability. Magnetic susceptibility of Ga (weak dia/paramagnetism) indicated negligible impact in operating temperatures. Prototype verification: A saddle coil (length 4.98 mm, inner diameter 3 mm) integrated with a 2.6 mm diameter detection chamber was fabricated (SAP). Unloaded Q factors for similarly structured 3D-printed NMR probeheads were 45–50. A deionized water 1H spectrum with single-scan FWHM of 26 Hz demonstrated usability. In situ EC-NMR probehead (ECP): A custom EC-NMR probehead integrated three channels/sockets (18 mm length, 4.3 mm diameter) for Pt working/counter electrodes and Ag reference electrode, converging at the NMR detection region to mitigate bubble interference. Pt electrodes were pretreated (Pt nanoparticle loading) via cyclic voltammetry. An ethanol oxidation reaction (500 µL aqueous; 1.0 M ethanol, 0.1 M HClO4) was driven at 0.9 V for up to 10 h while acquiring time-resolved 1H NMR (ID pulse sequence; 10 µs pulse, 57 dBm RF, 2 s delay, 3 s acquisition, 8 scans). Peaks were referenced/quantified using TSP. Continuous-flow separation probehead (CFSP): A modified reaction monitoring probehead incorporated microfluidic modules: spiral mixing/reaction channel (1.8 mm channel diameter; 15 mm spiral diameter; 15 mm height; 3-turn solenoid), a precipitation filter chamber (15 mm height, 25 mm diameter) packed with silica gel (250–830 µm) retained by cotton and a 0.8 mm outlet, an ion separation channel (1.8 mm diameter; 2.5 mm height; 1-turn solenoid), and a Y-fork outlet with sample/waste channels. A saddle coil focused on a 13 µL detection region. The system monitored isopropyl alcohol oxidation by KMnO4 in neutral solution. Silica gel adsorbed MnO2 precipitates and some paramagnetic ions; remaining Mn2+/K+ ions were separated by Lorentz-force effects in the strong vertical B0 field, diverting ion-rich streams to waste while delivering cleaner sample to detection. COMSOL CFD (laminar flow with dilute species transport) confirmed efficient mixing up to 100 µL min^-1. Flow rates of 20–100 µL min^-1 set residence times of 10–2 min. Delay times between scans were scaled to ensure full volume refresh. Mn2+ concentrations at outlets were measured with a manganese ion meter for validation. Comparative experiments without separation used a commercial 5 mm probe (unloaded Q=160).

- Integrative 3D-printed MR probeheads embedding LM-infused RF coils and customized sample/microfluidic chambers were successfully fabricated with micrometer precision (printing resolution 25–30 µm; coil channel diameters down to 400 µm). - Material screening identified VisiJet M3 Crystal (PLA-like) as the preferred substrate due to low εr (2.68), low loss (tanδ=0.012), and highest unloaded Q for LM-filled saddle coils (Q≈45) among tested printable materials. - LM optimization: Dispersing Au microparticles into Ga increased conductivity, peaking at 3 wt% Au with 3.82×10^6 S m^-1 (~10% above pure Ga). Conductivity was stable across 293–308 K (<5% variation). Solidification was improved by particle-induced nucleation, enhancing thermal tolerance. - RF simulations (500 MHz) showed saddle coils outperform modified Alderman-Grant coils: saddle Q=172, RF field uniformity ΔB=±4% B in the sample area, normalized SNR=1.0 versus AG Q=111, ΔB=±8% B, SNR=0.61; saddle coils were easier to tune at high frequency. - Prototype performance: 3D-printed LM saddle probeheads exhibited unloaded Q≈45–50; a single-scan water spectrum had FWHM 26 Hz with rough shimming, demonstrating practicality. - In situ EC-NMR: The integrated three-electrode ECP enabled real-time monitoring of ethanol oxidation at 0.9 V. 1H NMR revealed decreasing ethanol peaks (1.08, 3.57 ppm) and increasing acetic acid peak (2.08 ppm). Quantification indicated rapid reaction within 2 h followed by rate decline, consistent with active-site poisoning; incomplete oxidation to acetic acid dominated over full oxidation to CO2. - CFSP continuous-flow separation: The integrated filtration (silica gel) and Lorentz-force ion separation effectively reduced paramagnetic particle/ion content, yielding narrower spectral lines and higher SNR/resolution than non-separated flow. Mn2+ separation efficiency (difference in outlet concentrations and spectral FWHM) increased with flow rate. Real-time reaction monitoring of isopropanol oxidation by KMnO4 was achieved with improved spectral quality. - Despite lower conductor conductivity, 3D-printed LM coils enabled NMR/MRI applications; however, Q factors (≈44) were lower than copper coils with identical geometry (≈175), aligning with expectations from skin-depth and conductivity analyses.

The integrative 3D printing plus LM infusion approach directly addresses the limitations of conventional MR coil fabrication by enabling precise, rapid, and customizable monolithic probeheads that match coil geometry to sample volumes and complex microfluidic layouts. This improves filling factor and usability for unconventional experiments (in situ electrochemistry, continuous-flow reaction monitoring, and small-object MRI). Material and LM optimization provided a practical balance between printability, electromagnetic performance, and MR compatibility, with PLA-like materials minimizing dielectric losses and background, and Ga-based LM pastes offering sufficient conductivity and thermal stability. RF simulations justified the choice of saddle coils for high-frequency operation with superior homogeneity and SNR predictions, which translated into functional prototypes. The EC-NMR experiments validated real-time monitoring capability and chemical specificity, while the CFSP demonstrated that integrating filtration and magnetic/flow-based ion separation within the probehead can mitigate paramagnetic line broadening, yielding clearer spectra and enabling continuous monitoring of reactions that generate paramagnetic species. Although Q factors lag behind copper-based coils, the method prioritizes customization and integration; performance sufficed for the demonstrated applications. The work suggests that with further improvements in conductor materials, post-processing, and 3D metal printing, integrative probeheads could approach conventional performance while retaining design flexibility.

The study presents a flexible, rapid, and precise method to design and fabricate integrative MR probeheads via high-resolution 3D printing combined with liquid metal filling. The approach enables embedding of micrometer-scale non-capacitive RF coils and customized sample/microfluidic chambers in monolithic structures, supporting routine and specialized MR experiments. Material screening identified suitable printable substrates with favorable dielectric properties, and LM pastes optimized with gold microparticles improved conductor performance and stability. Simulations and experiments validated coil designs and demonstrated practical applications: in situ electrochemical reaction monitoring and continuous-flow reaction monitoring with on-probe separation of paramagnetic species, as well as small-sample MRI. Future work should focus on enhancing Q factors and SNR through improved conductor formulations (e.g., incorporating silver nanowires), refined channel cleaning/filling, optimized coil geometries, and adoption of advanced 3D metal printing to reduce resistive losses and eliminate auxiliary components. These developments could broaden the adoption of customized integrative probeheads in NMR studies and clinical MRI.

- Lower Q factors of LM-based coils (≈44–50) compared to copper coils of identical geometry (≈175) due to lower conductivity, reducing ultimate sensitivity. - Commercial confidentiality limited full disclosure of some building material compositions; generalizability of measured dielectric properties may vary across suppliers/batches. - LM coils can partially melt under prolonged RF duty cycles or exothermic reactions; although mixed particles improve solidification and thermal tolerance, careful temperature control is required. - Non-capacitive coil constraint limits some coil design options; tuning at very high frequencies remains challenging for certain geometries. - Background signals and hydrogen content of materials must be managed; while PLA-like material performed well, other materials may introduce spectral baseline artifacts. - Continuous-flow separation performance depends on flow rates, packing uniformity of silica gel, and channel fabrication fidelity; residual paramagnetic species can still broaden lines as reactions proceed.